This invention relates to a polarization diversity optical heterodyne receiver with demodulation and subsequent combination of two intermediate frequency (I.F.) signals.
An optical heterodyne detection communication (coherent optical communication) network is advantageous in long-distance and high-frequency-density transmission of signal beams because of its high reception sensitivity and its high frequency utilization capability in the manner described in an article contributed by Yoshihisa Yamamoto et al to the IEEE Journal of Quantum Electronics, Volume QE-17, No. 6 (June 1981), pages 919 to 935, under the title of "Coherent Optical Fiber Transmission Systems". When used in the network, a receiver is supplied with a signal beam carrying information and comprises a local optical source for generating a local beam, a beam coupler for coupling the signal and the local beams into a coupled or mixed beam, an optical detector or photodetector for detecting the coupled beam to produce an intermediate frequency signal, and a demodulator for demodulating the intermediate frequency signal to produce a reproduction of the information as a baseband signal.
It is inevitable that the signal beam is undesiredly subjected to fluctuation in its polarization state upon arrival at the receiver. As a result, the polarization state of the signal beam is not necessarily coincident with the polarization state of the local beam. This adversely affects the reception sensitivity. Furthermore, the local beam has an optical frequency which is subject to variation. This adversely affects reception characteristics or performance of the receiver. It is therefore mandatory in the receiver to compensate for the polarization fluctuation of the signal beam and to stabilize the optical frequency of the local beam.
Polarization diversity reception is effective in an optical heterodyne receiver in compensating for the polarization fluctuation of the signal beam. A polarization diversity optical heterodyne receiver comprises a beam splitting part supplied with the signal and the local beams for producing first and second coupled beams which are orthogonally polarized, a first optical detector for detecting the first coupled beam to produce a first intermediate frequency signal, a second optical detector for detecting the second coupled beam to produce a second intermediate frequency signal, and a processing part for processing the first and the second intermediate frequency signals into the baseband signal which is stable against the polarization fluctuation.
In the processing part, the first and the second intermediate frequency signals may first be combined into a combined intermediate frequency signal and then demodulated into the baseband signal. It is necessary in this event to preliminarily phase adjust the first and the second intermediate frequency signals for subsequent combination. Alternatively, the first and the second intermediate frequency signals may first be demodulated into first and second demodulated signals for subsequent combination into the baseband signal.
A processing part for subjecting the first and the second intermediate frequency signals to demodulation combination, namely, to demodulation and subsequent combination, is advantageous because the receiver is simple in structure and has a less deteriorated reception sensitivity. The simple structure is evident from unnecessity of a phase adjuster for preliminarily phase adjusting the first and the second intermediate frequency signals before supply to the processing part. The less deteriorated reception sensitivity is described, for example, in an article contributed by B. Glance to the Journal of Lightwave Technology, Volume 5 (1987), page 274, under the title of "Polarization Independent Coherent Optical Receiver". Glance theoretically shows in his article that a deterioration in the reception sensitivity is 0.4 dB in a polarization diversity optical heterodyne receiver in which combination follows demodulation carried out according to differential PSK detection.
In the polarization diversity optical heterodyne receiver, the frequency variation in the local beam results in a frequency variation of the first and the second intermediate frequency signals. In order to frequency stabilize the local beam, a frequency discriminating device is used in frequency discriminating the first and the second intermediate frequency signals to produce a control signal for use in the local optical source in controlling the optical frequency. For frequency discrimination, the first and the second intermediate frequency signals may preliminarily be combined into a single combined intermediate frequency signal. It is to be noted in this connection that each of the first and the second intermediate frequency signals has a phase and a power level which are unavoidably subjected to fluctuation due to the polarization fluctuation of the signal beam. As a consequence, the combined intermediate frequency signal disappears in a worst case where the first and the second intermediate frequency signals cancel each other. It is therefore mandatory to use two frequency discriminators in the frequency discriminating device. This undesiredly renders the polarization diversity optical heterodyne receiver bulky and expensive.